Vacuum arc devices with doubly reentrant coaxial arc-electrode structure



April .28,1970

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April 28, 1970 J. A. RICH 3,509,404

VACUUM ARC DEVICES WITH DOUBLY REENTRANT COAXIAL ARC-ELECTRODE STRUCTURE Filed July l, 1968 5 Sheets-Sheet 2 Fig. Z.

April 28,r 1970 J. A. RICH 3 509,404

9 VACUUM ARC DEVICES WITH DOUBLY REENTRANT COAXIAL ARC-ELECTRODE STRUCTURE FiledJuly 1, 1968 3 Sheets-Sheet 5 .777 Venter: Joseph AMR/ch,

is Attorney.

United States Patent O 3,509,404 VACUUM ARC DEVICES WITH DOUBLY REENTRANT COAXIAL ARC-ELECTRODE STRUCTURE Joseph A. Rich, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed July 1, 1968, Ser. No. 741,480 Int. Cl. H01h 32/12;H01j17/30, 2]/22 U.S. 'CL 313-148 10 Claims ABSTRACT F THE DISCLOSURE arc rotation without exposure thereof to arcing current paths.

The present invention relates to vacuum arc devices, particularly devices of the triggerable vacuum gap and vacuum switch types, having uniquely-formed arc-electrode structures facilitating the attainment of high-arcing currents without the formation of destructive anode spots. More particularly, the present invention relates to such devices wherein a pair of coaxial electrode assemblies are formed in reentrant geometry to achieve the foregoing ends.

The present invention is related to my copending application Ser. No. 639,693, led May 19, 1967, and my concurrently-filed applications, Ser Nos. 741,481 and 741,482, all of which are assigned to the present assignee, and which are incorporated herein by reference thereto.

In vacuum arc devices such as triggerable vacuum gap devices and vacuum switches, a limitation upon the magnitude of current which may be carried is the formation of destructive anode spots which cause melting of the anodes. In the prior art, attempts have been made to avoid such anode spot formation by provision of broad-area arc-electrodes and by the incorporation of electrode structures which cause arc rotation. In general, their approaches have reached a point of diminishing return.

In another approach, I have discovered and have set forth in my aforementioned copending application, Ser. No. 639,693, that a prime factor in the formation of anode spots is the bunching of arc conduction paths between large area arc-electrodes by the orthogonal force given by the vector product of the current density with the magnetic field caused by current within the arc-electrode structures. Accordingly, in that application, I have provided reentrant inner arc-electrode structures which greatly minimize the azimuthal magnetic eld between coaxial arc-electrode structures and minimize the forces which tend to bunch arc-conduction paths and form arcelectrode, particularly anode, spots. While devices constructed in accord with my aforementioned copending application greatly increase the current carrying capacity 0f vacuum arc devices, it is desirable that such current conduction paths may be enhanced and the tolerable currents accommodated by a given geometry or size arcelectrode device may be increased. One method of doing this is to incorporate means for rotating the current conduction paths which do form to further diminish the task and one which is dicult to achieve, particularly ice in the environment of the coaxial reentrant arc-electrode structures provided in accord with my aforementioned copending application.

Accordingly, it is an object of the present invention to provide improved vacuum arc devices including methods for providing arc rotation.

Yet another object of the present invention is to provide vacuum are devices having reentrant arc-electrode structures including means for causing arc rotation.

Still another object of the present invention is to provide reentrant arc electrode structures for vacuum arc devices having integral arc rotation means.

Briefly stated, in accord with the one embodiment of the present invention, I provide vacuum arc devices such as triggerable vacuum gaps and vacuum switches having a pair of coaxial reentrant arc-electrode structures. The inner of the coaxial arc-electrode assemblies is reentrant to minimize the azimuthal field within the arcing gap. The outer of the coaxial arc-electrode assemblies is reentrant in order that one cylindrical portion thereof may include a multilar helix which causes the creation of a solenoidal magnetic eld within the arcing gap to cause arc rotation, while the inner cylindrical member thereof forms a surface for the establishment of footpoints of arc conduction paths thereon which shields the slots of the multiilar helix of the outer cylindrical member of the electrode assembly and prevents the preferential formation of arc footpoint at the sharp edges thereof.

The novel features characteristic of the present invention are set forth in the appended claims. The invention itself, together with `further objects and advantages thereof, may best be understood by reference to the appended drawing in which:

-FIGURE 1 is a vertical cross-sectional view of a triggerable vacuum gap device constructed in accord with one embodiment of the present invention,

FIGURE 2 is a vertical cross-sectional view of a vacuum switch device constructed in accord with yet another embodiment of the present invention, and

FIGURE 3 is a plan sectional view of the device of FIGURE 2.

In FIGURE 1, a triggerable vacuum gap device constructed in accord with the present invention, and represented generally, as 10, includes an hermetically sealed envelope 11 evacuated to a pressure of 10-5 torr or less and comprising sidewall member 11 of a suitable insulating material as, for example, Pyrex, or Nonex glass or a suitable ceramic such as high-density alumina, and a pair of oppositely-disposed endwall members or assemblies 13 and 14, hermetically sealed to opposite ends of sidewall member 11 to form an evacuable envelope. Within evacuable envelope 11, a pair of rentrant cylindrical arc-electrode assemblies 15 and 16 are disposed coaxially about the longitudinal axis of envelope 11 with inner arc-electrode assembly 16 depending into the major portion of outer arc-electrode assembly 15 to form a hollow, cylindrical, primary arcing gap 17, which comprises the major portion of the longitudinal dimension of device 10.

A trigger assembly 18 is disposed within a flanged aperture in endwall member 14, aligned with a portion of primary arcing gap 17, and is adapted, when pulsed with a suitable voltage pulse, to cause the injection of an electron-ion plasma into the primary arcing gap to cause the device to change abruptly from a non-conductive to a conductive state. A pair or terminal lugs 19 and 20 are integrally mechanically and electrically axed to endwall members 13 and 14, respectively, and comprise means for connecting device 10 to a circuit or circuit component which is to be switched, protected, or otherwise controlled by the action thereof.

`Outer coaxial arc-electrode assembly 15 includes a rst outer cylinder 21, which is mechanically and electrically aflixed to endwall member 13, and is parallel with and adjacent sidewall member 12, Assembly also includes a first inner cylinder 22 which is not connected with endwall member 13, but rather, is affixed at the inwardlydepending end thereof to the inwardly-depending end of first outer cylinder 21 by a first bridging member 23. Inner coaxial electrode assembly 16 includes a second inner cylinder 24 which is electrically and mechanically affixed to endwall member 14, and a second outer cylinder 25 which is not directly affixed to endwall member 14, but rather, is affixed at the inwardly-depending end thereof to the inwardly-depending end of second inner cylinder 24 by a second bridging member 26, to form a reentrant cylindrical structure. The inner cylinder 22 of outer coaxial arc-electrode assembly 15 and the outer cylinder 25 of inner cylindrical arc-electrode assembly 1'6 define therebetween a hollow cylindrical arcing gap 17 which has a longitudinal dimension greater than half the length of either arc-electrode assembly and greater than half the longitudinal dimension of device 10, and which is adapted to be filled with electron-ion plasma upon breakdown thereof with a multiplicity of arclets represented, generally, by simulated arcs 27 between the inner surface of the outer electrode assembly and the outer surface of the inner electrode assembly. For complete insulation, it is preferable that insulating sidewall member 12 extend the length of the entire interelectrode gap 17.

The outer cylinder 21 of outer coaxial arc-electrode assembly 15 is slotted with a plurality of parallel helical slots 28 to form a mu'ltifilar helix. The helical structure causes currents within the reentrant portion 22 of arcelectrode assembly 15, all of which passes through outer cylindrical member 21, to follow a helical path, producing a solenoidal magnetic lfield having a substantially longitudinal configuration, within primary arc gap 17. Since the devices of the present invention are contemplated to be used at exceedingly high currents of the order of tens of thousands of amperes at tens of thousands of volts, the forces which act upon the arc-electrode structures, due to the magnetic forces therein and the weight of the arc-electrode assembly itself, are substantial. Accordingly, arc-electrode structures in the form of helices must not be flexible, such as would be the case if a continuous single helical structure were utilized, as in some devices of the prior art, to cause arc rotation. I have found that a substantial amount of arc rotation is achieved by virtue of slotting of the outer cylinder of outer coaxial electrode assembly 15, when each slot transverses no more than approximately 180 of the cylindrical surface thereof in passing from the uppermost portion to the lowermost portion thereof. Such structure allows also for a sufficient rigidity, so that no distortion or deformation of the arc-electrode assembly is caused by the weight of the arc-electrode assembly and the forces generated during arcing #at these currents and voltages.

Ioperationally, the folded configuration of the reentrant coaxial inner electrode structure 16 produces a substantial cancellation of the azimuthal component of the magnetic field caused by currents therein. Thus, electrode currents due to arcing currents between cylinders 22 and 25 must pass in one direction within cylinder 25 and in the opposite direction within cylinder 24. To the extent that the oppositely-directed currents within adjacent portions of cylinders 24 and 25 are equal, the azimuthal magnetic field existing within primary arc gap 17 thereat is zero. This is the condition which exists at region A thereof, where substantially all of the arc-conduction current existing between the arc-electrode assemblies exists both in the portions of cylinders 24 and 25 at region A. Such cancellation occurs to a lesser extent at region B thereof, where a lesser amount of current exists in that portion of cylinder 25 than exists in cylinder 24. In general, however, I nd that the moderate inward concentration of the arc-conduction paths within primary arcing gap 17 by the slight unbalance in region B is not suicient to cause undue bunching and arc conduction paths are substantially evenly distributed over the inwardly-depending portion, greater than half, of arc-electrode assembly 16, with a very low current density, in relation to the total current therein, and a greatly increased threshold for the formation of destructive anode spots.

A further increase in the threshold for the formation of destructive anode spots is achieved by causing the arcconduction paths within primary arcing gap 17 to rotate under the influence of the longitudinal component of the magnetic field strength represented by H which is substantially orthogonal to current conduction paths 27, causing a rotational force acting normally to the plane of the drawing and a rotation about the inner concentric arc-electrode assembly.

The dimensions of the device, as illustrated in FIG- URE 1, vary with the arc currents which are desired to be carried thereby and the operating voltage. As the currents and voltages are increased, the diameter of both the inner and outer arc-electrode assemblies is increased so as to increase the area of the hollow cylindrical arcing gap 17. The radial dimension of the arcing gap is not very critical, since the dielectric strength of vacuum is sufficiently high so as to preclude the danger of dielectric breakdown for reasonable gap lengths.

As an example, however, of devices which may be constructed in accord with the present invention, one such device as illustrated in FIGURE 1 utilized a Pyrex glass sidewall member 12 having a radial wall thickness of 2%; inch and a length and an interior diameter each of 6 inches. The thickness of the walls of both reentrant c0- axial arc-electrode assemblies was 1/s inch, while the outer diameter of cylinder 21 was 5 inches, the inner diameter of cylinder 22 was 4 inches, the outer diameter of cylinder 25 was 3 inches, and the diameter of inner cylinder 24 was 11/2 inches. Eight slots 28 were cut in exterior cylinder 21 of arc-electrode assembly 21, the slots being approximately 1/2 inch on center and 1/8 inch in width, the length of the entire multifilar helix formed thereby being 4 inches. The arcing gap 17 had a length of 4 inches and a radial thickness of 1/2 inch. When trigger electrode assembly 18 was pulsed with a 200 volt pulse of 5 microseconds duration and a capacitor bank charged to 10 kilovolts was applied across terminals 19 and 20, a peak current of 50,000 amperes having a 60- cycle sine wave form was carried by the arc-electrode assemblies for typical arc-interruption periods of several cycles or less without the formation of destructive anode spots and the attendant melting thereof. Repeated breakdown did not form anode-melting spots.

Arc-electrode assemblies 15 and 16, since they are to be subjected to high currents and heated to high temperatures, must be free of all gases and gas-forming impurities in order that the evolution of gases therefrom during arcing does not adversely affect the quiescent vacuum within envelope 11. Although the vacuum does not exist during arcing, due to the evolution of a large quantity of metallic specie from the arc-electrodes, it is essential, upon arc interruption, that the metal be able to condense out, leaving a residual pressure of the order of 10*5 torr or less, in order that the high dielectric strength of vacuum be operative to hold off the high voltage applied between the arc-electrode assemblies. Thus, the arc- 'electrodes assemblies are formed from a metal as, for

example, copper, beryllium, alloys therebetween, or any of the materials set forth in Patents 2,975,236, Lee et al.; 2,975,255, Lafferty; 3,016,435, Lafferty; 3,140,373, Horn; and 3,246,979, Lafferty et al., for example. Preferably these materials are prepared in high purity form as, for example, by multi-pass zone refining or by a special zone refining technique such as is set forth in Patent No. 3,234,351, Hebb.

Trigger assembly 18 comprises a scored metal or a hydride film formed upon an insulating ceramic disc 29,

the outer portion of which serves as trigger cathode and is connected to sidewall member 14, and a trigger anode which is a hydride or metallic coated ceramic post 28, having the upper end thereof coated and connected with a wire which runs centrally therethrough and emerges as a trigger lead 31, connected to a pulse source 32. Such triggers may conveniently be the triggers disclosed in Lafferty application, Ser. No. 564,132, filed Iuly 1l, 1966, or alternatively, may be that disclosed in Lafferty application, Ser. No. 704,935, filed Feb. 12, 1968, both of which are assigned to the present assignee.

FIGURE 2 of the drawing illustrates a vacuum switch constructed in accord with another embodiment of the present invention. In FIGURE 2, vacuum switch 40 includes an evacuable envelope 41 comprising an insulating sidewall member 42, hermetically sealed to, and electrically providing insulation between, an upper endwall assembly 43 and a lower endwall assembly 44. Lower endwall assembly 44 includes a transverse member 45 and a cylindrical longitudinal mem-ber `46 which is inwardly depending.

Within envelope 41, an outer coaxial arc-electrode assembly 47, similar to arc-electrode assembly 15 of FIGURE l, is inwardly depending from endwall assembly 43 and includes a first helically slotted outer cylinder 49 and a first inner cylinder 50 joined at their inwardlydepending ends With a first bridging member 51 having a substantially flat contact-making surface S2, the entire electrode assembly ybeing affixed to endwall assembly 43 at the outwardly-depending end of cylinder 49.

An inner concentric arc-electrode assembly 48 includes a base member 53 which extends transversely across the end of a sidewall member 41 and is hermetically sealed between one end thereof and the upwardly-depending sidewall member 46 of lower endwall assembly 44. A second inner cylinder 54 is electrically and mechanically afiixed to base member 53, and is connected to a second outer cylinder 55 at the inwardly-depending end thereby by a bridging member 56. Inner coaxial arc-electrode assembly 48 is electrically connected to a contact making means 57 by a iiexible metallic conductor 58, electrically connected therebetween. Contact-making assembly 57 includes a transverse plate member 59 and a plurality, either two, three, or four, for example, of discrete upwardlydepending contact members 60 which pass through apertures 61 in base member 53 of arc-electrode assembly 48 and terminate in contact surfaces `62, which are adapted to mate with, and be disconnected from, discrete portions of surface 52 of first bridging member 51 of outer coaxial arc-electrode 47 to facilitate the initiation of a starting arc therebetween, when it is desired to change the device from a stable current-conducting condition to an intermediate, arcing, circuit-interrupting condition.

Contact-making assembly 57 is longitudinally movable from a circuit-making condition, in which surfaces 52 and 62 are abutted together, to a circuit-breaking position, in which contacts 52 and 62 are separated, by longitudinal4 movement of an actuating rod 63, hermetically sealed, and electrically connected, to transverse member 45 of end-wall assembly 44 by a flexible bellows 64. Flexible bellows 64 is adapted to allow sufiicient longitudinal motion so as to permit the separation of surfaces 52 and 62 to a greater distance than the distance between the inner surface of cylinder 50 and the outer surface of cylinder S, so as to aid magnetic transfer of the initial Starting arc from relatively limited area surfaces 52 and 62 to the broad area of the primary arcing gap 67 between the outer coaxial arc-electrode assembly and the inner coaxial arc-electrode assembly. Such magnetic transfer is due to the rotational effect of the magnetic forces upon arc conduction paths.

In operation, the vacuum switch 40 may be connected in series with a circuit or circuit component to be protected from overload transients by connection with terminal lug 65, integral with end-wall assembly 43 and terminal lug 66, integral with endwall assembly 44. Upon the occurrence of a transient overload, operating rod 63 is withdrawn downwardly, separating surfaces 52 and 62 and initiating an arc-conduction path therebetween. As the distance between surfaces 52 and 62 exceeds the distance of primary arcing gap 67, the arc footpoint on surface 62 is transferred to the outer surface of cylinder member and the primary arc gap 67 with its broad area assumes the burden of the high current arc which spreads substantially uniformly thereover as in the device of FIGURE l. Upon the occurrence of a current zero value, the arc is interrupted and the circuit opened.

Alternatively, the initiation of arc-conduction between outer arc-electrode assembly 47 and inner arc-electrode assembly 48 may be initiated by a pulse of electron-ion plasma from a suitable trigger assembly, such as is illustrated at 18 in FIGURE l of the drawing, which may be located adjacent the primary arcing gaps 67 through endwall assembly 43 or may be adjacent the starting contact points 52 and 6-2 through endwall assembly 44. This may be accomplished when the switch is in a circuit-open position, and allows the device to become conducting rapidly, while the mechanical closure mechanism is causing the arc-electrode assemblies to become abutted.

FIGURE 3 of the drawing illustrates a horizontal plan view taken along section line 3 3 in FIGURE 2 of the drawing and illustrates the manner in which contact members (herein chosen to be three in number) protrude through apertures 161 in `base member 53, so as to make contact with discrete portions of surface 52 of first bridging member 51 of outer coaxial arc-electrode assembly 47. As is illustrated herein, trigger assembly 68 is adapted to inject electron-ion plasma into the vicinity of the arcing gap through endwall assembly 44 to stabilize or fix, in time, the instant at which the arc current between the normally open coaxial electrodes is initiated.

More specifically, a trigger pulse may be instituted when the device 40 is in a normally open position and it is desired to cause protection of a circuit or circuit component by short-circuiting a circuit or circuit component with which it is connected in parallel circuit relationship. In such an instance the device is normally in a circuit-open, non-conducting state and, upon the occurrence of the transient over-voltage from which it is desired to protect the circuit or circuit component, a pulse of electron-ion plasma is injected between the arc-electrodes into gap 67 to cause electrically an initial conduction path to relieve the arc transient, which arc-conduction path may be established within a matter of microseconds, utilizing the trigger pulse. Simultaneously, the actuating arm 63 may mechanically cause contact assembly 57 to place switch 40 in the circuit-closed position with surfaces 52 and y62 abutting, to carry the overload current.

Devices in accord with the present invention are greatly use ful in that they provide the advantages of the nested coaxial arc-electrode structure with minimum azimuthal magnetic field in the arc gap, and the advantage of the broad area arc gap to attain low-current density and the increased threshold for the formation of anode spots. Additionally, devices in accord with the present invention have the advantages of further raising the threshold for the formation of anode spots by virtue of magnetic rotation of arc-conduction paths within the primary arc-gap in a manner which is consistant with shielding of slotted members, to provide such a magnetic eld `while protecting the field-producing member from the prmary are to prevent the formation of arc footprints on the sharp edges thereof. Devices in accord with the present invention may be constructed in a number of ways. Thus, for example, a single trigger may be utilized as is illustrated in FIGURE 1 or a plurality of triggers either through the same endwall member or through the sidewall member may be utilized to initiate the arcing currents. Additionally, with the device as illustrated in FIGURE 2, as shown in FIGURE 3, a trigger may be incorporated together with means for movably connecting one electrode with the other to cause the device to move from a circuit-open to a circuit-closed position.

Although the invention has been described herein rwith respect to certain speciiic embodiments and examples thereof, many mociiications and changes will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A vacuum arc device comprising:

(a) an hermetically `sealed envelope evacuated to a vacuum of l5 torr or less and including (a1) rst and second Oppositely disposed endwall assemblies, and (a2) an insulating sidewall member hermetically sealed to and electrically isolating said first and second endwall assemblies (b) a iirst, outer reentrant coaxial arc-electrode assembly including,

(bl) a first outer cylinder electrically connected to one of said endwall assemblies (b2) a irst inner cylinder coaxial with said first outer cylinder and not directly connected to said one endwall assembly, and

(b3) a first bridging member connecting said first inner and said rst outer cylinder at the inwardly-depending ends thereof to form a rigid reentrant cylindrical assembly,

(b4) said first outer cylinder being slotted by a plurality of helical slots around the circumference thereof forming a rigid multifilar helical structure which establishes a solenoidal magnetic field within said device upon the conduction of current therein;

(c) a second inner reentrant coaxial arc-electrode assembly including,

(c1) a second inner cylinder electrically connected to the second of said endwall assemblies,

(c2) a second outer cylinder concentric with said second inner cylinder, not directly connected with said second endwall assembly, joined by a second bridging member to said second inner cylinder at the inwardly-depending ends thereof and disposed with said rst inner cylinder to form a primary arcing gap;

(d) means for connecting said device to an electric circuit to be protected, switched, or controlled; and (e) means for providing an electron-ion plasma within said primary arcing gap at a predetermined time when it is desired to render said device conducting. 2. The device of claim 1 wherein said inner arc-elec- `trode assembly is extended into the volume of said outer arc-electrode assembly by an amount greater than one half thereof.

3. The device of claim 1 wherein said insulating sidewall member is greater in logitudinal extent than the longitudinal overlap of said arc-electrode assemblies.

4. The device of claim 1 wherein said helical slots extend around the periphery of said first outer cylinder an azimuthal span of no greater than approximately 5. The device of claim 1 wherein said means for establishing an electron-ion plasma within said envelope is a trigger gap assembly and said primary arc-electrodes are both fixed.

6. The device of claim 1 wherein a contact making portion one of said arc-electrode assemblies is adapted to move longitudinally with respect to the other of said arc-electrode assemblies between a circuit-closed and a circuit-open position and includes contact-making means for abutting against said other primary arc-electrode assembly, said contact making means constituting means for providing an electron-ion plasma within said envelope.

7. The device of claim 6 wherein a trigger gap assembly is further juxtaposed adjacent said primary arcing gap between said outer and said inner arc-electrode assemblies for injecting an electron-ion plasma therebetween at a predetermined time.

8. The device of claim 1 where said primary arcing gap is the closet distance between any portion of said first and second primary arc-electrode members.

9. The device of claim 1 Where said primary arcing gap has the geometry of a hollow cylinder.

10. The device of claim 1 where said primary portion of said arc-electrode assembly is longitudinally movable a sulicient distance as to permit the contact-making means to be separated from any portion of said other primary arc-electrode assembly by a distance which is greater than the dimension of said primary arcing gap.

References Cited UNITED STATES PATENTS 3,246,979 4/1966 Lafferty et al. 200-144 X 3,320,478 5/1967 Harrison 313-155 X 3,328,632 6/1967 Robinson '1313-233 X 3,331,981 7/1967 Laferty 313-233 X 3,417,116 12/1968 Smith 200-144 3,450,922 6/1969 Gallagher 313-155 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner U.S. Cl. X.R. 

